It is well known in the plastics molding art to produce molded articles which have the appearance of decorative stone such as marble. Often referred to as "cultured marble" these plastic molded articles can be manufactured for a fraction of the cost associated with real marble and have been accepted by consumers as aesthetically pleasing. A number of specific techniques have been used in the past to achieve the color striations or swirls in the surface layer of the plastic which approximates the look of real marble. Generally, these processes include the use of contrasting colorants, one for the resin matrix to give the base color and others which are added to produce the contrasting striations. By limiting the degree to which the striation colorants are admixed with the base resin, the marble appearance can be obtained.
In the manufacture of large molded articles such as sinks, bathtubs and lavatories, the molding process is typically carried out using the so-called "gel coat" process. In the gel coat process, a mold having a predetermined configuration is initially coated with a substantially clear, hardenable resin coating. This type of initial coating is referred to as the gel coat. After the gel coat is applied, a first layer of a resin and filler mixture is sprayed over the gel coat. Spraying has generally taken the place of the prior practice of casting although many gel coats do not adhere well to the mold when sprayed. Striations of opaque filler are then formed in the first layer of the resin/filler mixture. After the striated resin/filler layer has partially cured so as to form a tacky coating a layer of glass fibers is disposed over the layer and laminated thereon. The glass fibers may be pre-treated with a coating of a hardenable resin and a catalyst and the fiber-resin filler composition is permitted to cure at least to a tacky state. By utilizing a fiber layer, a high strength to weight ratio is achieved. A second layer of resin/filler mixture is applied over the glass fibers and permitted to cure. After final and total curing of the various layers has occurred, the product is removed from the mold. Prior art gel coat systems typically form a gel coat of from about 20-30 mils.
One type of molding process used for this purpose is resin transfer molding. In this process, the male mold is coated with the gel coat, the preform is dropped into the female mold, the mold is closed and the resin is injected into the mold cavity.
It is also known to use a gel coat which incorporates materials to produce the desired simulated stone appearance. For example, in PCT International publication No. wo 93/09170, a process of forming a plastic article which has the appearance of granite is disclosed in which the gel coat includes plastic granules. More specifically, granules made from the same resin as the gel coat, or a compatible resin, are mixed into the gel coat resin. In order to obtain even dispersion and suspension of the granules in the gel coat it is specified that they must have the same specific gravity as the gel coat resin matrix. The gel coat further contains aluminum trihydrate. A pigment/filler is used to mask the coloration of the backing material which is applied in the mold. The size of the aluminum trihydrate granules is disclosed as 20 microns average grain granule diameter. It is further disclosed therein that the alumina trihydrate constitutes up to about 50% by weight of the resin and that thixotropes may be added.
The molded articles produced using the prior art gel coat techniques suffer from a number of drawbacks such as problems associated with the differences in the coefficients of thermal expansion between the surface coating and the underlying materials. Particularly in the case of bath tubs, hot tubs, and lavatories and the like, this differential in thermal expansion can cause crazing and other surface distortions as the surface expands at a different rate than the underlying material. In addition the nature of conventional surface layers makes these materials difficult to repair where a scratch or crack develops on the surface of the article. The present invention is directed to solving these and other drawbacks associated with prior art techniques.